1. Field of the Invention
The present invention relates to optical disk drives, and more particularly, to an optical disk drive provided with means for preventing an inner reference clock signal from being disturbed due to a partial missing of a read reproduction signal caused by a defect on a disk medium.
2. Description of the Related Art
In optical disk drives for reproducing information recorded on a disk medium such as a DVD-ROM, in general, to extract data from a bit reproduction (RF) signal (clock signal mixed with data) read from the disk medium, an inner reference clock signal generated inside the optical disk drives is synchronized in phase and in frequency with the RF signal and the data is extracted with the use of a window signal formed from the inner reference clock signal.
In this case, such an optical disk drive is provided with a phase-locked loop (PLL) circuit having a voltage-controlled oscillator (VCO), a phase comparator, and a low-pass filter (LPF). In addition, a frequency comparator is also provided in order to speed up the synchronization of the PLL circuit to the RF signal and to adjust the inner reference clock signal so as to be always in the best condition. In this optical disk drive, when the frequency of a read RF signal varies, the frequency change is detected by the frequency comparator, a frequency error signal is sent to the PLL circuit, and the frequency of the inner reference clock signal is adjusted to the best condition. In this way, the synchronization of the inner reference clock signal with the RF signal is sped up and then data can be accurately extracted.
FIG. 3 is a block diagram showing an example structure of a main section of a known optical disk drive performing such an operation.
As shown in FIG. 3, this known optical disk drive is provided with a VCO 31 constituting a controllable free-running oscillator, a phase comparator 32, an LPF 33, a frequency comparator 34, a data-latch circuit 35, a window-signal generation section 36, an RF-signal input terminal 37, a data output terminal 38, and an inner-reference-clock-signal output terminal 39. In this case, the VCO 31, the phase comparator 32, and the LPF 33 form a PLL circuit 30.
One input end of the VCO 31 is connected to the output end of the LPF 33, the other input end is connected to the output end of the frequency comparator 34, and the output end is connected to the input end of the window-signal generation section 36 and to the inner-reference-clock-signal output terminal 39. One input end of the phase comparator 32 is connected to an RF-signal input terminal 37, the other input end is connected to the output end of the VCO 31, and the output end is connected to the input end of the LPF 33. One input end of the frequency comparator 34 is connected to the RF-signal input terminal 37 and the other input end is connected to the output end of the VCO 31. The latch end of the data-latch circuit 35 is connected to the output end of the window-signal generation signal 36, the input end is connected to the RF-signal input terminal 37, and the output end is connected to the data output terminal 38.
The known optical disk drive having the above structure schematically operates in the following way.
The RF signal obtained by reading a disk medium is sent to the phase comparator 32, the frequency comparator 34, and the data-latch circuit 35 through the RF-signal input terminal 37. The phase comparator 32 compares the inner reference clock signal generated by the VCO 31 with a clock signal included in the RF signal, generates a phase error signal indicating a phase difference therebetween, and sends it to the LPF 33. The LPF 33 smoothes the phase error signal, converts it to a phase error voltage, and sends the voltage to the VCO 31. The frequency comparator 34 compares in frequency the inner reference clock signal generated by the VCO 31 with the clock signal included in the RF signal, generates a frequency error signal indicating a frequency difference therebetween, and sends it to the VCO 31.
In the VCO 31, the phase of the inner reference clock signal is adjusted in response to the sent phase error voltage, and the frequency of the inner reference clock signal is adjusted so as to be a predetermined frequency in response to the sent frequency error signal. In this way, the inner reference clock signal output from the VCO 31 is adjusted so as to be synchronized in phase and in frequency with the clock signal included in the RF signal.
The inner reference clock signal output from the VCO 31 is sent to the inner-reference-clock-signal output terminal 39, and is also converted to a window signal to be generated when data in the RF signal reaches, in the window-signal generation section 36, and is sent to the data-latch circuit 35. The data-latch circuit 35 reads the data when the window signal is sent, namely, when the data in the sent RF signal reaches, and sends the read data to the data output terminal 38.
In the known optical disk drive, the PLL circuit 30 formed of the VCO 31, the phase comparator 32, and the LPF 33 synchronizes in phase the inner reference clock signal output from the VCO 31 with the clock signal included in the RF signal. In a closed control loop formed of the VCO 31 and the frequency comparator 34, the frequency of the inner reference clock signal output from the VCO 31 is adjusted so as to promptly become equal to the frequency of the clock signal included in the RF signal.
When the known optical disk drive reads a disk medium having a defect described later, in other words, when the defective portion on the disk medium is read, such as a scrape and a scratch on the reading surface of the disk medium, a bit defect and a missing bit (interruption) on the bit recording surface of the disk medium, or a stain (black dot), such as that formed by a magic marker, which is stuck to the reading surface of the disk medium and does not reflect light, an RF signal having an inaccurate clock signal is output or an RF signal having no clock signal is output. Therefore, the frequency error signal output from the frequency comparator 34 becomes extremely large or extremely small, and the inner reference clock signal output from the VCO 31 has an inappropriate frequency largely deviated from the appropriate frequency. When the frequency of the inner reference clock signal output from the VCO 31 is largely deviated from the appropriate frequency, phase synchronization is not performed in the PLL 30 and the inner reference clock signal output from the VCO 31 becomes runaway.
Once the inner reference clock signal goes runaway, even when a normal RF signal having the clock signal arrives again when the defective portion on the disk medium has been read, it takes a long time for the inner reference clock signal output from the VCO 31 to have the appropriate frequency.
As described above, in the known optical disk drive, when a disk medium having a defect is read, the frequency of the inner reference clock signal output from the VCO 31 is temporarily deviated from the appropriate frequency greatly, and during that period, data in the RF signal cannot be read by the data-latch circuit 35.
To solve this drawback, there has already been proposed a disk drive in which a missing portion of an RF signal is detected according to the envelope of the RF signal, and when the missing portion is detected, the frequency of the inner reference clock signal output from the VCO is maintained at the frequency used immediately before the missing portion is detected to prevent the frequency of the inner reference clock signal from being greatly deviated; and a disk drive, as disclosed by the Unexamined Patent Publication No. Hei-10-69734, in which the switching means is opened due to a missing detection signal obtained by detecting a missing portion of the RF signal to maintain the frequency of the inner reference clock signal output from the VCO at the frequency used immediately before the missing detection signal is sent, by the use of an holding circuit, and the time constant of the holding circuit is switched when the missing detection signal is sent to narrow the capture range of the PLL circuit.
In the above two disk drives proposed, however, only the PLL circuit is used to control the frequency of the inner reference clock signal output from the VCO. The disk drive does not operate such that the frequency and the phase of the inner reference clock signal are adjusted by the use of the frequency comparator and the PLL circuit.
Since the technical means employed by the two proposed disk drives, that is, phase-synchronization-releasing preventing means for the PLL circuit, cannot be applied as is to a disk drive in which the frequency and the phase of the inner reference clock signal are adjusted by the use of the frequency comparator and the PLL circuit, new technical means has been demanded.
Accordingly, it is an object of the present invention to provide an optical disk drive which allows the frequency and the phase of an inner reference clock signal to be promptly compensated for their shifts after a defective portion on a disk medium has been read when the frequency and the phase of the inner reference clock signal are adjusted with the use of a frequency comparator and a PLL circuit.
The foregoing object is achieved according to the present invention through the provision of an optical disk drive including an voltage-controlled oscillator (VCO) for generating an inner reference clock signal; a data extraction section for extracting data from a reproduction (RF) signal read from a disk medium with the use of the inner reference clock signal; a phase comparator for comparing in phase the RF signal with the inner reference clock signal to send a phase error signal to the VCO; a frequency comparator for comparing in frequency the RF signal with the inner reference clock signal to send a frequency error signal to the VCO through a switching unit; and a defect detection section for detecting a defect on the disk medium from the envelope of the RF signal to output a defect detection signal, wherein the switching unit is switched so as to break a connection between the VCO and the frequency comparator when the defect detection signal is output.
The optical disk drive may be configured such that the switching unit is a controllable switch in which a make and a break are switched by the defect detection signal.
The optical disk drive may be configured such that the switching unit includes a controllable switch in which a make and a break are switched by the defect detection signal and a voltage holding unit connected to one of movable contacts of the controllable switch, for holding the voltage level of the frequency error signal, and the voltage held by the voltage holding unit is sent to the VCO through the controllable switch when the defect detection signal is output.
According to the present invention, when a defect on the disk medium is read and the defect detection section outputs a defect detection signal, the switching unit is opened with the use of the defect detection signal to break a connection between the VCO and the frequency comparator, and the transmission of the frequency error signal to the VCO is blocked. Therefore, even if a large disturbance is generated in the frequency error signal output from the frequency comparator due to missing of the clock signal in the RF signal or inaccuracy of the clock signal when the defective portion is read on the disk medium, the disturbed frequency error signal is not sent to the VCO and the frequency of the inner reference clock signal output from the VCO is maintained almost at the same state as that used immediately before the defect detection signal is output.
In addition, according to the present invention, when the defective portion of the disk medium has been read and the output of the defect detection signal from the defect detection section is stopped, the switching unit is again closed to connect the VCO to the frequency comparator, and an RF signal having the normal clock signal is sent to the frequency comparator. Therefore, an appropriate frequency error signal output from the frequency comparator is sent to the VCO, and the inner reference clock signal output from the VCO is immediately re-synchronized in frequency with the clock signal in the RF signal. The data extraction section can extract data from the RF signal. A period of time from when the defective portion of the disk medium has been read to when data is extracted is greatly reduced compared with this type of known apparatuses.